1. Field of the Invention
The present invention concerns a switching-over automatic matching filter for radio transmitter and/or receiver stations.
It can be applied notably to the making of frequency hop switching-over filters.
2. Description of the Prior Art
In prior art techniques for making radio frequency filters, the need to obtain operation that requires only low current consumption, without any modification of the matching frequency of the filter in the presence of an interfering signal, is contradictory to the need to ensure high agility in frequency.
A first category of filters are the filters matched by values of capacitances, switched over by means of PIN diodes. To ensure high agility in frequency, the switching-over operations are done at the matched circuits. However, high current values are used to bias the diodes, in order to avoid damping the resonant circuits of the filter by high resistance values and ensure high immunity to interfering signals.
A second category of filters ar the filters matched by levels of capacitances, switched over by electromechanical relays. These filters consume a great deal of energy through the frequent modifying of their matching frequency, and do not provide for high agility in frequency.
A third category of filters are the band-pass filters, switched over by PIN diodes. These filters call for a great number of band-pass cells to achieve good selectivity. Like the filters of the previous category, they consume a great deal of current, and have the drawback of taking up a great amount of space, in terms of volume, in the equipment.
Finally the fourth category are the filters matched by variable capacitance diodes which permit high agility in frequency with low consumption. Unfortunately, when there are interfering signals caused by nearby transmitting stations, the bias voltage of the variable capacitance diodes is disturbed, causing the mismatching of the filters.